1. Field of the Invention
The present invention relates to aromatic and heteroaromatic compounds with nonlinear optical (NLO) properties. In particular, the present invention relates to NLO compounds having improved chemical and thermal stability.
The compounds of the present invention are stable in processing solvents and at processing temperatures used in the production of electro-optic devices. When suitably oriented, the compounds are capable of highly efficient second harmonic generation and electro-optic modulation of an electromagnetic wave having a wavelength between 300 nm and 2,000 nm. The present invention further relates to polymer compositions of the disclosed compounds.
2. Description of the Prior Art
Highly efficient NLO materials capable of doubling or tripling the frequency of incident light are currently of great scientific and technological interest for use in optical telecommunications, signal processing and the construction of optical computers. Nonlinear optics is concerned with the interaction of electromagnetic fields in various media to produce new fields which may be altered in phase, frequency or amplitude. The NLO effect of a material upon an electromagnetic field is a function of the second and higher order terms of the following equation: EQU P=.alpha.E+.beta.E.sup.2 .gamma.E.sup.3 + . . .
P is the polarization of a material, E is the intensity of electric field, and the coefficients .alpha., .beta. and .gamma., etc. are indicative of the NLO susceptibility of the material. Such coefficients are constant for a given material, but vary from material to material. The second order coefficient, .beta., for a given material, is indicative of the second harmonic generation properties of the material, with second harmonic generation efficiencies increasing as the value of .beta. increases.
Candidate NLO materials should possess good physical properties, such as high optical transparency, low dielectric constant and high laser damage threshold. The materials should also possess the molecular nonlinearity required of NLO materials, in particular, high .beta. values, fast response times and nonlinear susceptibility over a broad range of wavelengths, particularly of wavelengths between about 300 nm and 2,000 nm.
Recent efforts in the development of NLO materials have focused upon non-centrosymmetric organic materials with large delocalized pi-electron systems, which exhibit great nonlinear susceptibilities and can be varied to optimize the desired physical and mechanical properties. This includes the single benzene ring derivative disclosed by U.S. Pat. No. 4,894,186 to Gordon and the compounds derived from two to four benzene rings separated by pi-electron conjugated carbon-carbon, carbon-nitrogen and nitrogen-nitrogen bridges disclosed by U.S. Pat. No. 4,892,681 to Myata et. al., U.S. Pat. No. 4,894,263 to Dubois et al., U.S. Pat. No. 4,933,112 to DeMartino et. al. and U.S. Pat. No. 4,935,292 to Marks et. al.
To induce charge asymmetry, and consequently second order nonlinear polarizability, an aromatic ring at one end of the NLO compound structure is substituted with an electron donating group, while on the other end of the NLO compound structure an aromatic ring is substituted with an electron accepting group. The dipole of the compound structure can then be aligned in accordance with the method described by U.S. Pat. No. 4,935,292, the disclosure of which is hereby incorporated herein by reference thereto.
Copending and commonly owned U.S. patent application Ser. No. 07/626,358 filed Dec. 12, 1990, discloses another group of NLO compounds containing from two to ten six-membered aromatic or five-membered heteroaromatic rings or fused ring systems linked together by the above-listed pi-electron conjugated bridges. At least one five-membered heteroaromatic ring is present by itself, or as part of a fused ring system, which heteroaromtic ring contains at least one heteroatom selected from O, N, S or Se. The five-membered heteroaromatic ring increases the extent of the electron delocalization associated with NLO activity and facilitates the enhancement of NLO activity as the number of aromatic rings increases. The disclosure of this patent application is hereby incorporated herein by reference thereto.
However, pi-electron conjugated bridges linking the aromatic or heteroaromatic rings of NLO compounds are a source of thermal and photochemical instability. This is addressed by copending and commonly owned U.S. patent application Ser. No. 07/930,732, filed Aug. 14, 1992, the disclosure of which is hereby incorporated herein by reference thereto. This application discloses NLO compounds derived from highly conjugated fused ring structures of two or three aromatic or heteroaromatic rings, at least one of which is a five-membered heteroaromatic ring. The pi-electron conjugated bridges are eliminated. This application also discloses NLO compounds derived from one to four non-fused five-membered heteroaromatic rings linked together without pi-conjugated bridges.
The stability of non-centrosymmetric organic materials with large delocalized pi-electron systems in processing solvents and host polymers at processing temperatures is an important parameter in their application in electro-optic devices. Because high-T.sub.g polyimide based electro-optic polymers are likely candidates to be used in the production of NLO devices, candidate NLO materials must survive the stringent processing conditions required to produce stable electro-optic polyimides. One of the commonly employed processing conditions requires the solubilization of candidate NLO materials in polyamic acid solutions using solvents such as N-methyl-pyrrolidone (NMP), dimethylacetamide (DMAC), dimethylformamide (DMF), and the like, and the curing of the resultant polymers at high temperatures (200.degree.-300.degree. C).
NLO compounds that are stable in solvents such as NMP, DMAC and DMF, both at room temperature and at boiling temperatures, are not well established in the literature. The general instability of NLO compounds in the solvents is attributable to the sensitivity of the electron accepting groups toward solvent decomposition products, typically open chain amine-type basic impurities induced by either light, oxygen or heat. Accordingly, a need exists for NLO compounds that are stable under polyimide curing and processing conditions.